Unique pattern of neutrophil migration and function during tumor progression.

Although neutrophils have been linked to the formation of the pre-metastatic niche, the mechanism of their migration to distant, uninvolved tissues has remained elusive. We report that bone marrow neutrophils from mice with early-stage cancer exhibited much more spontaneous migration than that of control neutrophils from tumor-free mice. These cells lacked immunosuppressive activity but had elevated rates of oxidative phosphorylation and glycolysis, and increased production of ATP, relative to that of control neutrophils. Their enhanced spontaneous migration was mediated by autocrine ATP signaling through purinergic receptors. In ectopic tumor models and late stages of cancer, bone marrow neutrophils demonstrated potent immunosuppressive activity. However, these cells had metabolic and migratory activity indistinguishable from that of control neutrophils. A similar pattern of migration was observed for neutrophils and polymorphonuclear myeloid-derived suppressor cells from patients with cancer. These results elucidate the dynamic changes that neutrophils undergo in cancer and demonstrate the mechanism of neutrophils' contribution to early tumor dissemination.

Ferroptosis of tumour neutrophils causes immune suppression in cancer.

Ferroptosis is a non-apoptotic form of regulated cell death that is triggered by the discoordination of regulatory redox mechanisms culminating in massive peroxidation of polyunsaturated phospholipids. Ferroptosis inducers have shown considerable effectiveness in killing tumour cells in vitro, yet there has been no obvious success in experimental animal models, with the notable exception of immunodeficient mice1,2. This suggests that the effect of ferroptosis on immune cells remains poorly understood. Pathologically activated neutrophils (PMNs), termed myeloid-derived suppressor cells (PMN-MDSCs), are major negative regulators of anti-tumour immunity3-5. Here we found that PMN-MDSCs in the tumour microenvironment spontaneously die by ferroptosis. Although decreasing the presence of PMN-MDSCs, ferroptosis induces the release of oxygenated lipids and limits the activity of human and mouse T cells. In immunocompetent mice, genetic and pharmacological inhibition of ferroptosis abrogates suppressive activity of PMN-MDSCs, reduces tumour progression and synergizes with immune checkpoint blockade to suppress the tumour growth. By contrast, induction of ferroptosis in immunocompetent mice promotes tumour growth. Thus, ferroptosis is a unique and targetable immunosuppressive mechanism of PMN-MDSCs in the tumour microenvironment that can be pharmacologically modulated to limit tumour progression.

Epigenetic silencing of retinoblastoma gene regulates pathologic differentiation of myeloid cells in cancer.

Two major populations of myeloid-derived suppressor cells (MDSCs), monocytic MDSCs (M-MDSCs) and polymorphonuclear MDSCs (PMN-MDSCs) regulate immune responses in cancer and other pathologic conditions. Under physiologic conditions, Ly6C(hi)Ly6G(-) inflammatory monocytes, which are the normal counterpart of M-MDSCs, differentiate into macrophages and dendritic cells. PMN-MDSCs are the predominant group of MDSCs that accumulates in cancer. Here we show that a large proportion of M-MDSCs in tumor-bearing mice acquired phenotypic, morphological and functional features of PMN-MDSCs. Acquisition of this phenotype, but not the functional attributes of PMN-MDSCs, was mediated by transcriptional silencing of the retinoblastoma gene through epigenetic modifications mediated by histone deacetylase 2 (HDAC-2). These data demonstrate a new regulatory mechanism of myeloid cells in cancer.

Fatty acid transport protein 2 reprograms neutrophils in cancer.

Polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) are pathologically activated neutrophils that are crucial for the regulation of immune responses in cancer. These cells contribute to the failure of cancer therapies and are associated with poor clinical outcomes. Despite recent advances in the understanding of PMN-MDSC biology, the mechanisms responsible for the pathological activation of neutrophils are not well defined, and this limits the selective targeting of these cells. Here we report that mouse and human PMN-MDSCs exclusively upregulate fatty acid transport protein 2 (FATP2). Overexpression of FATP2 in PMN-MDSCs was controlled by granulocyte-macrophage colony-stimulating factor, through the activation of the STAT5 transcription factor. Deletion of FATP2 abrogated the suppressive activity of PMN-MDSCs. The main mechanism of FATP2-mediated suppressive activity involved the uptake of arachidonic acid and the synthesis of prostaglandin E2. The selective pharmacological inhibition of FATP2 abrogated the activity of PMN-MDSCs and substantially delayed tumour progression. In combination with checkpoint inhibitors, FATP2 inhibition blocked tumour progression in mice. Thus, FATP2 mediates the acquisition of immunosuppressive activity by PMN-MDSCs and represents a target to inhibit the functions of PMN-MDSCs selectively and to improve the efficiency of cancer therapy.

Defective Epstein-Barr Virus Genomes and Atypical Viral Gene Expression in B-Cell Lines Derived from Multiple Myeloma Patients.

Epstein-Barr virus (EBV) is a human gammaherpesvirus that is causally associated with various lymphomas and carcinomas. Although EBV is not typically associated with multiple myeloma (MM), it can be found in some B-cell lines derived from MM patients. Here, we analyzed two EBV-positive MM-patient-derived cell lines, IM9 and ARH77, and found defective viral genomes and atypical viral gene expression patterns. We performed transcriptome sequencing to characterize the viral and cellular properties of the two EBV-positive cell lines, compared to the canonical MM cell line 8226. Principal-component analyses indicated that IM9 and ARH77 clustered together and distinct from 8226. Immunological Genome Project analysis designated these cells as stem cell and bone marrow derived. IM9 and ARH77 displayed atypical viral gene expression, including leaky lytic cycle gene expression with an absence of lytic DNA amplification. Genome sequencing revealed that the EBV genomes in ARH77 contain large deletions, while IM9 has copy number losses in multiple EBV loci. Both IM9 and ARH77 showed EBV genome heterogeneity, suggesting cells harboring multiple and variant viral genomes. We identified atypical high-level expression of lytic genes BLRF1 and BLRF2. We demonstrated that short hairpin RNA (shRNA) depletion of BLRF2 altered viral and host gene expression, including a reduction in lytic gene activation and DNA amplification. These findings demonstrate that aberrant viral genomes and lytic gene expression persist in rare B cells derived from MM tumors, and they suggest that EBV may contribute to the etiology of MM. IMPORTANCE EBV is an oncogenic herpesvirus, but its mechanisms of oncogenesis are not fully understood. A role for EBV in MM has not yet been established. We analyzed EBV-positive B-cell lines derived from MM patients and found that the cells harbored defective viral genomes with aberrant viral gene expression patterns and cell gene signatures for bone marrow-derived lymphoid stem cells. These findings suggest that aberrant EBV latent infection may contribute to the etiology of MM.

Neutrophils and PMN-MDSC: Their biological role and interaction with stromal cells.

Neutrophils and polymorphonucler myeloid-derived suppressor cells (PMN-MDSC) share origin and many morphological and phenotypic features. However, they have different biological role. Neutrophils are one of the major mechanisms of protection against invading pathogens, whereas PMN-MDSC have immune suppressive activity and restrict immune responses in cancer, chronic infectious disease, trauma, sepsis, and many other pathological conditions. Although in healthy adult individuals, PMN-MDSC are not or barely detectable, in patients with cancer and many other diseases they accumulate at various degree and co-exist with neutrophils. Recent advances allow for better distinction of these cells and better understanding of their biological role. Accumulating evidence indicates PMN-MDSC as pathologically activated neutrophils, with important role in regulation of immune responses. In this review, we provide an overview on the definition and characterization of PMN-MDSC and neutrophils, their pathological significance in a variety of diseases, and their interaction with other stromal components.

Bone marrow myeloid cells in regulation of multiple myeloma progression.

Survival, growth, and response to chemotherapy of cancer cells depends strongly on the interaction of cancer cells with the tumor microenvironment. In multiple myeloma, a cancer of plasma cells that localizes preferentially in the bone marrow, the microenvironment is highly enriched with myeloid cells. The majority of myeloid cells are represented by mature and immature neutrophils. The contribution of the different myeloid cell populations to tumor progression and chemoresistance in multiple myeloma is discussed.

Regulation of dendritic cell differentiation in bone marrow during emergency myelopoiesis.

Although accumulation of dendritic cell (DC) precursors occurs in bone marrow, the terminal differentiation of these cells takes place outside bone marrow. The signaling, regulating this process, remains poorly understood. We demonstrated that this process could be differentially regulated by Notch ligands: Jagged-1 (Jag1) and Delta-like ligand 1 (Dll1). In contrast to Dll1, Jag1, in vitro and during induced myelopoiesis in vivo, prevented DC differentiation by promoting the accumulation of their precursors. Although both ligands activated Notch in hematopoietic progenitor cells, they had an opposite effect on Wnt signaling. Dll1 activated Wnt pathways, whereas Jag1 inhibited it via downregulation of the expression of the Wnt receptors Frizzled (Fzd). Jag1 suppressed fzd expression by retaining histone deacetylase 1 in the complex with the transcription factor CSL/CBF-1 on the fzd promoter. Our results suggest that DC differentiation, during induced myelopoiesis, can be regulated by the nature of the Notch ligand expressed on adjacent stroma cells.

Myeloid-derived suppressor cells regulate growth of multiple myeloma by inhibiting T cells in bone marrow.

Myeloid-derived suppressor cells (MDSC) are one of the major factors limiting the immune response in cancer. However, their role in bone marrow (BM), the site of primary localization of multiple myeloma (MM), is poorly understood. In this study, we found a significant accumulation of CD11b(+)CD14(-)CD33(+) immunosuppressive MDSC in BM of patients with newly diagnosed MM. To assess the possible role of MDSC in MM, we used immunocompetent mouse models. Immunosuppressive MDSC accumulated in BM of mice as early as 1 wk after tumor inoculation. S100A9 knockout (KO) mice, which are deficient in their ability to accumulate MDSC in tumor-bearing hosts, demonstrated reduced MDSC accumulation in BM after injection of MM cells compared with wild-type mice. Growth of the immunogenic MM cells was significantly reduced in S100A9KO mice. This effect was associated with the accumulation of Ag-specific CD8(+) T cells in BM and spleens of S100A9KO mice, but not wild-type mice, and was abrogated by the administration of anti-CD8 Ab or adoptive transfer of MDSC. Thus, the accumulation of MDSC at early stages of MM plays a critical role in MM progression and suggests that MDSC can be considered a possible therapeutic target in this disease.

Intraperitoneal activation of myeloid cells clears ascites and reveals IL27-dependent regression of metastatic ovarian cancer.

Patients with metastatic ovarian cancer (OvCa) have a 5-year survival rate of less than 30% due to persisting dissemination of chemoresistant cells in the peritoneal fluid and the immunosuppressive microenvironment in the peritoneal cavity. Here, we report that intraperitoneal administration of ß-glucan and IFNγ (BI) induced robust tumor regression in clinically relevant models of metastatic OvCa. BI induced tumor regression by controlling fluid tumor burden and activating localized antitumor immunity. ß-glucan alone cleared ascites and eliminated fluid tumor cells by inducing intraperitoneal clotting in the fluid and Dectin-1-Syk-dependent NETosis in the omentum. In omentum tumors, BI expanded a novel subset of immunostimulatory IL27+ macrophages and neutralizing IL27 impaired BI efficacy in vivo. Moreover, BI directly induced IL27 secretion in macrophages where single agent treatment did not. Finally, BI extended mouse survival in a chemoresistant model and significantly improved chemotherapy response in a chemo-sensitive model. In summary, we propose a new therapeutic strategy for the treatment of metastatic OvCa.

Rational design of shepherdin, a novel anticancer agent.

Anticancer agents that selectively kill tumor cells and spare normal tissues are urgently needed. Here, we engineered a cell-permeable peptidomimetic, shepherdin, modeled on the binding interface between the molecular chaperone Hsp90 and the antiapoptotic and mitotic regulator, survivin. Shepherdin makes extensive contacts with the ATP pocket of Hsp90, destabilizes its client proteins, and induces massive death of tumor cells by apoptotic and nonapoptotic mechanisms. Conversely, shepherdin does not reduce the viability of normal cells, and does not affect colony formation of purified hematopoietic progenitors. Systemic administration of shepherdin in vivo is well tolerated, and inhibits human tumor growth in mice without toxicity. Shepherdin could provide a potent and selective anticancer agent in humans.

Intercellular hif1α reprograms mammary progenitors and myeloid immune evasion to drive high-risk breast lesions.

The origin of breast cancer, whether primary or recurrent, is unknown. Here, we show that invasive breast cancer cells exposed to hypoxia release small extracellular vesicles (sEVs) that disrupt the differentiation of normal mammary epithelia, expand stem and luminal progenitor cells, and induce atypical ductal hyperplasia and intraepithelial neoplasia. This was accompanied by systemic immunosuppression with increased myeloid cell release of the alarmin S100A9 and oncogenic traits of epithelial-mesenchymal transition, angiogenesis, and local and disseminated luminal cell invasion in vivo. In the presence of a mammary gland driver oncogene (MMTV-PyMT), hypoxic sEVs accelerated bilateral breast cancer onset and progression. Mechanistically, genetic or pharmacologic targeting of hypoxia-inducible factor-1α (HIF1α) packaged in hypoxic sEVs or homozygous deletion of S100A9 normalized mammary gland differentiation, restored T cell function, and prevented atypical hyperplasia. The transcriptome of sEV-induced mammary gland lesions resembled luminal breast cancer, and detection of HIF1α in plasma circulating sEVs from luminal breast cancer patients correlated with disease recurrence. Therefore, sEV-HIF1α signaling drives both local and systemic mechanisms of mammary gland transformation at high risk for evolution to multifocal breast cancer. This pathway may provide a readily accessible biomarker of luminal breast cancer progression.

Redox phospholipidomics discovers pro-ferroptotic death signals in A375 melanoma cells in vitro and in vivo.

Growing cancer cells effectively evade most programs of regulated cell death, particularly apoptosis. This necessitates a search for alternative therapeutic modalities to cause cancer cell's demise, among them - ferroptosis. One of the obstacles to using pro-ferroptotic agents to treat cancer is the lack of adequate biomarkers of ferroptosis. Ferroptosis is accompanied by peroxidation of polyunsaturated species of phosphatidylethanolamine (PE) to hydroperoxy- (-OOH) derivatives, which act as death signals. We demonstrate that RSL3-induced death of A375 melanoma cells in vitro was fully preventable by ferrostatin-1, suggesting their high susceptibility to ferroptosis. Treatment of A375 cells with RSL3 caused a significant accumulation of PE-(18:0/20:4-OOH) and PE-(18:0/22:4-OOH), the biomarkers of ferroptosis, as well as oxidatively truncated products - PE-(18:0/hydroxy-8-oxo-oct-6-enoic acid (HOOA) and PC-(18:0/HOOA). A significant suppressive effect of RSL3 on melanoma growth was observed in vivo (utilizing a xenograft model of inoculation of GFP-labeled A375 cells into immune-deficient athymic nude mice). Redox phospholipidomics revealed elevated levels of 18:0/20:4-OOH in RSL3-treated group vs controls. In addition, PE-(18:0/20:4-OOH) species were identified as major contributors to the separation of control and RSL3-treated groups, with the highest variable importance in projection predictive score. Pearson correlation analysis revealed an association between tumor weight and contents of PE-(18:0/20:4-OOH) (r = -0.505), PE-18:0/HOOA (r = -0.547) and PE 16:0-HOOA (r = -0.503). Thus, LC-MS/MS based redox lipidomics is a sensitive and precise approach for the detection and characterization of phospholipid biomarkers of ferroptosis induced in cancer cells by radio- and chemotherapy.

Syntaphilin Regulates Neutrophil Migration in Cancer.

Pathologically activated neutrophils (PMN) with immunosuppressive activity, which are termed myeloid-derived suppressor cells (PMN-MDSC), play a critical role in regulating tumor progression. These cells have been implicated in promoting tumor metastases by contributing to premetastatic niche formation. This effect was facilitated by enhanced spontaneous migration of PMN from bone marrow to the premetastatic niches during the early-stage of cancer development. The molecular mechanisms underpinning this phenomenon remained unclear. In this study, we found that syntaphilin (SNPH), a cytoskeletal protein previously known for anchoring mitochondria to the microtubule in neurons and tumor cells, could regulate migration of PMN. Expression of SNPH was decreased in PMN from tumor-bearing mice and patients with cancer as compared with PMN from tumor-free mice and healthy donors, respectively. In Snph-knockout (SNPH-KO) mice, spontaneous migration of PMN was increased and the mice showed increased metastasis. Mechanistically, in SNPH-KO mice, the speed and distance travelled by mitochondria in PMN was increased, rates of oxidative phosphorylation and glycolysis were elevated, and generation of adenosine was increased. Thus, our study reveals a molecular mechanism regulating increased migratory activity of PMN during cancer progression and suggests a novel therapeutic targeting opportunity.

S100A8/S100A9 Promote Progression of Multiple Myeloma via Expansion of Megakaryocytes.

Multiple myeloma is characterized by clonal proliferation of plasma cells that accumulate preferentially in the bone marrow (BM). The tumor microenvironment is one of the leading factors that promote tumor progression. Neutrophils and monocytes are a major part of the BM tumor microenvironment, but the mechanism of their contribution to multiple myeloma progression remains unclear. Here, we describe a novel mechanism by which S100A8/S100A9 proteins produced by BM neutrophils and monocytes promote the expansion of megakaryocytes supporting multiple myeloma progression. S100A8/S100A9 alone was not sufficient to drive megakaryopoiesis but markedly enhanced the effect of thrombopoietin, an effect that was mediated by Toll-like receptor 4 and activation of the STAT5 transcription factor. Targeting S100A9 with tasquinimod as a single agent and in combination with lenalidomide and with proteasome inhibitors has potent antimyeloma effect that is at least partly independent of the adaptive immune system. This newly identified axis of signaling involving myeloid cells and megakaryocytes may provide a new avenue for therapeutic targeting in multiple myeloma. Significance: We identified a novel mechanism by which myeloid cells promote myeloma progression independently of the adaptive immune system. Specifically, we discovered a novel role of S100A8/S100A9, the most abundant proteins produced by neutrophils and monocytes, in regulation of myeloma progression via promotion of the megakaryocyte expansion and angiogenesis. Tasquinimod, an inhibitor of S100A9, has potent antimyeloma effects as a single agent and in combination with lenalidomide and with proteasome inhibitors.

An African-Specific Variant of TP53 Reveals PADI4 as a Regulator of p53-Mediated Tumor Suppression.

TP53 is the most frequently mutated gene in cancer, yet key target genes for p53-mediated tumor suppression remain unidentified. Here, we characterize a rare, African-specific germline variant of TP53 in the DNA-binding domain Tyr107His (Y107H). Nuclear magnetic resonance and crystal structures reveal that Y107H is structurally similar to wild-type p53. Consistent with this, we find that Y107H can suppress tumor colony formation and is impaired for the transactivation of only a small subset of p53 target genes; this includes the epigenetic modifier PADI4, which deiminates arginine to the nonnatural amino acid citrulline. Surprisingly, we show that Y107H mice develop spontaneous cancers and metastases and that Y107H shows impaired tumor suppression in two other models. We show that PADI4 is itself tumor suppressive and that it requires an intact immune system for tumor suppression. We identify a p53-PADI4 gene signature that is predictive of survival and the efficacy of immune-checkpoint inhibitors. SIGNIFICANCE: We analyze the African-centric Y107H hypomorphic variant and show that it confers increased cancer risk; we use Y107H in order to identify PADI4 as a key tumor-suppressive p53 target gene that contributes to an immune modulation signature and that is predictive of cancer survival and the success of immunotherapy. See related commentary by Bhatta and Cooks, p. 1518. This article is highlighted in the In This Issue feature, p. 1501.

Nitrogen-Doped Carbon Nanotube Cups for Cancer Therapy.

Carbon nanomaterials have attracted significant attention for a variety of biomedical applications including sensing and detection, photothermal therapy, and delivery of therapeutic cargo. The ease of chemical functionalization, tunable length scales and morphologies, and ability to undergo complete enzymatic degradation make carbon nanomaterials an ideal drug delivery system. Much work has been done to synthesize carbon nanomaterials ranging from carbon dots, graphene, and carbon nanotubes to carbon nanocapsules, specifically carbon nanohorns or nitrogen-doped carbon nanocups. Here, we analyze specific properties of nitrogen-doped carbon nanotube cups which have been designed and utilized as drug delivery systems with the focus on the loading of these nanocapsules with specific therapeutic cargo and the targeted delivery for cancer therapy. We also summarize our targeted synthesis of gold nanoparticles on the open edge of nitrogen-doped carbon nanotube cups to create loaded and sealed nanocarriers for the delivery of chemotherapeutic agents to myeloid regulatory cells responsible for the immunosuppressive properties of the tumor microenvironment and thus tumor immune escape.

Protection of Regulatory T Cells from Fragility and Inactivation in the Tumor Microenvironment.

Fragility of regulatory T (Treg) cells manifested by the loss of neuropilin-1 (NRP1) and expression of IFNγ undermines the immune suppressive functions of Treg cells and contributes to the success of immune therapies against cancers. Intratumoral Treg cells somehow avoid fragility; however, the mechanisms by which Treg cells are protected from fragility in the tumor microenvironment are not well understood. Here, we demonstrate that the IFNAR1 chain of the type I IFN (IFN1) receptor was downregulated on intratumoral Treg cells. Downregulation of IFNAR1 mediated by p38α kinase protected Treg cells from fragility and maintained NRP1 levels, which were decreased in response to IFN1. Genetic or pharmacologic inactivation of p38α and stabilization of IFNAR1 in Treg cells induced fragility and inhibited their immune suppressive and protumorigenic activities. The inhibitor of sumoylation TAK981 (Subasumstat) upregulated IFNAR1, eliciting Treg fragility and inhibiting tumor growth in an IFNAR1-dependent manner. These findings describe a mechanism by which intratumoral Treg cells retain immunosuppressive activities and suggest therapeutic approaches for inducing Treg fragility and increasing the efficacy of immunotherapies.

A Novel Selective Inhibitor JBI-589 Targets PAD4-Mediated Neutrophil Migration to Suppress Tumor Progression.

Neutrophils are closely involved in the regulation of tumor progression and formation of premetastatic niches. However, the mechanisms of their involvement and therapeutic regulation of these processes remain elusive. Here, we report a critical role of neutrophil peptidylarginine deiminase 4 (PAD4) in neutrophil migration in cancer. In several transplantable and genetically engineered mouse models, tumor growth was accompanied by significantly elevated enzymatic activity of neutrophil PAD4. Targeted deletion of PAD4 in neutrophils markedly decreased the intratumoral abundance of neutrophils and led to delayed growth of primary tumors and dramatically reduced lung metastases. PAD4-mediated neutrophil accumulation by regulating the expression of the major chemokine receptor CXCR2. PAD4 expression and activity as well as CXCR2 expression were significantly upregulated in neutrophils from patients with lung and colon cancers compared with healthy donors, and PAD4 and CXCR2 expression were positively correlated in neutrophils from patients with cancer. In tumor-bearing mice, pharmacologic inhibition of PAD4 with the novel PAD4 isoform-selective small molecule inhibitor JBI-589 resulted in reduced CXCR2 expression and blocked neutrophil chemotaxis. In mouse tumor models, targeted deletion of PAD4 in neutrophils or pharmacologic inhibition of PAD4 with JBI-589 reduced both primary tumor growth and lung metastases and substantially enhanced the effect of immune checkpoint inhibitors. Taken together, these results suggest a therapeutic potential of targeting PAD4 in cancer. SIGNIFICANCE: PAD4 regulates tumor progression by promoting neutrophil migration and can be targeted with a small molecule inhibitor to suppress tumor growth and metastasis and increase efficacy of immune checkpoint blockade therapy.

Immune suppressive activity of myeloid-derived suppressor cells in cancer requires inactivation of the type I interferon pathway.

Myeloid-derived suppressor cells (MDSC) are pathologically activated neutrophils and monocytes with potent immune suppressive activity. These cells play an important role in accelerating tumor progression and undermining the efficacy of anti-cancer therapies. The natural mechanisms limiting MDSC activity are not well understood. Here, we present evidence that type I interferons (IFN1) receptor signaling serves as a universal mechanism that restricts acquisition of suppressive activity by these cells. Downregulation of the IFNAR1 chain of this receptor is found in MDSC from cancer patients and mouse tumor models. The decrease in IFNAR1 depends on the activation of the p38 protein kinase and is required for activation of the immune suppressive phenotype. Whereas deletion of IFNAR1 is not sufficient to convert neutrophils and monocytes to MDSC, genetic stabilization of IFNAR1 in tumor bearing mice undermines suppressive activity of MDSC and has potent antitumor effect. Stabilizing IFNAR1 using inhibitor of p38 combined with the interferon induction therapy elicits a robust anti-tumor effect. Thus, negative regulatory mechanisms of MDSC function can be exploited therapeutically.